1. Field of the Invention
The present invention relates to a photoelectric conversion device including a window layer formed using an inorganic compound.
2. Description of the Related Art
In recent years, photoelectric conversion devices that do not produce carbon dioxide during power generation have attracted attention as a measure against global warming. As typical examples thereof, bulk-type solar cells which use crystalline silicon substrates such as single crystalline and polycrystalline silicon substrates and thin-film type solar cells which use a thin film such as an amorphous silicon film or a microcrystalline silicon film have been known.
In solar cells using a silicon substrate, a structure having a so-called homo junction is widely used. In such a structure, a layer having a conductivity type opposite to that of the silicon substrate is formed on one surface side of the silicon substrate by diffusion of impurities. Alternatively, a structure with a heterojunction is known in which amorphous silicon having different optical band gap and conductivity type from those of a silicon substrate is formed on one surface side of the silicon substrate (see Patent Documents 1 and 2).
A thin-film type solar cell includes a thin film which is formed using a required amount of silicon by a plasma CVD method or the like; thus, resource saving can be achieved as compared to the case of a bulk type solar cell. Further, by using a laser processing method, a screen printing method, or the like, the thin-film solar type cells can be easily formed in an integral manner and a large area of solar cells can be easily obtained; thus, manufacturing cost thereof can be reduced. However, the thin-film type solar cells have a disadvantage in lower conversion efficiency than the bulk-type solar cells.
In order to improve the conversion efficiency of the thin-film type solar cell, a method in which silicon oxide is used instead of silicon for a p-type semiconductor layer serving as a window layer has been disclosed (for example, see Patent Document 3). A non-single-crystal silicon based p-type semiconductor layer formed as a thin film is doped with an impurity and thus has a higher light absorption property than an i-type semiconductor layer which is a light absorption layer, which causes light loss caused by light absorption. An object of a technique disclosed in Patent Document 1 is to suppress light loss caused by light absorption in a window layer by using silicon oxide having a larger optical band gap than silicon for a p-type semiconductor layer.